1. Field of the Invention
The present invention relates to a digital receiver and, more particularly, to a technique that converts a received analog radio frequency (RF) signal into an intermediate frequency (IF) signal or a direct conversion (DC) signal by using a subsampling scheme and performs over-sampling on a desired signal band in order to convert even noise signals adjacent to a desired signal into a digital signal, thus digitally processing noise signals adjacent to the desired signal.
2. Description of the Related Art
FIG. 1 illustrates the structure of a conventional wireless communications receiver, in particular, a direct conversion (DC) receiver among those with various structures. In general, the conventional wireless communications receiver band-filters a signal received via an antenna, amplifies it, converts the amplified signal into a low-frequency band signal by using a mixer, filters a desired channel signal, and processes it through a variable gain amplifier (VGA) so that an analog-to-digital converter (ADC) can receive the signal having a certain magnitude. Namely, the conventional analog type receiver must filter out the undesired interference signal until such time as the analog signal is converted into the digital signal by using the ADC, which, thus, requires the mixer, the filter, and the VGA. These blocks, namely, the mixer, the filter, and the VGA, require a great deal of time to be designed and must be re-designed each time a process is upgraded. Thus, the use of the conventional analog designing scheme in developing a wireless transceiver such that is able to process multi-band signals and is applicable for various application fields is disadvantageous in terms of power consumption, chip area, and fast market adaptability.
Meanwhile, a wireless transceiver including digital design factors may complement the shortcomings of the analog designing scheme but it is difficult to implement such wireless transceiver including digital design factors.
In particular, in the case of a digital receiver that directly samples a high frequency band signal to perform digital signal processing, an ADC must operate at a considerably high frequency and have a high bit resolution, so it is not available to be implemented with the current technology. FIG. 2 is a conceptual view of an ideal software defined radio (SDR) receiver which filters a high frequency signal, amplifies it, and then immediately converts the amplified signal into a digital signal through an oversampling ADC. However, the receiver structure illustrated in FIG. 2 is merely a concept which cannot be implemented by current technology when a signal band is carried on a high carrier frequency. This is because the sampling frequency of the ADC should be at least double that of a carrier signal in order to satisfy a Nyquist theorem to restore a signal. For example, in order to process a signal of 2 GHz, an ADC which can operate at a sampling frequency of 4 GHz is needed, and in order to support a large input signal magnitude as well as operation speed, the ADC must have a large dynamic range. Also, if such an ADC was somehow to be implemented, the data output rate of the ADC would be so high that a digital processor at a rear stage of the ADC could not operate, and even if the digital processor managed to somehow operate, the issue of huge power consumption would remain to be handled.
Thus, in order to process a signal of a high frequency band, a conventional digital receiver must necessarily include a mixer for lowering the frequency band of the signal at a front stage of the ADC, a filter for canceling noise, and a VGA for adjusting the gain of the signal in order to obtain a signal of a certain magnitude. With these analog signal processing blocks, the ADC can only receive a signal as desired as possible and can also only receive a signal having as uniform a magnitude as possible, in order that the ADC can be easily designed.
FIG. 3 illustrates the concept and problem of sub sampling, showing a method for converting a signal having a high frequency into a signal having a low frequency through subsampling. Compared with a general Niquist sampling scheme, all signals located at positions corresponding to the multiplicity of sampling frequencies (fs) overlap with finally sampled signals by aliasing through subsampling. Thus, with this simple scheme, it is virtually impossible to obtain a desired signal-to-noise ratio at a final signal. Thus, in general, for both a Niquist sampling scheme and a sub sampling scheme, an anti-aliasing filter should be necessarily positioned at a front stage of the ADC.
FIG. 4 illustrates the structure of a digital receiver using a discrete signal processor, in which the digital receiver using a discrete signal processor may be an intermediary between the existing analog type receiver that can be implemented and an ideal digital receiver. In the digital receiver illustrated in FIG. 4, after a signal is filtered and amplified, it is processed by a discrete signal processor. Namely, the digital receiver illustrated in FIG. 4 has such a simple structure by significantly reducing the burden of the required filter and VGA. However, although the digital receiver has a modified structure for processing a signal in a discrete time domain, the signal remains an analog signal, which is thus still far too weak to obtain many advantages that can be otherwise obtained when a perfect digital receiver is implemented.